AN4457
Application note
Implementing an emulated universal asynchronous receiver
transmitter on STM32F4 microcontroller series
Introduction
This application note describes how to implement an emulated universal asynchronous
receiver transmitter (UART) on STM32F4 series.
Such an emulator is needed in applications that require more UARTs than the ones provided
by STM32F4 microcontrollers.
The emulated UART is full-duplex, supports up to 9 data bits and baud rates up to
115200 bps. It also offers a high flexibility since any I/O pin can be used as TX or RX line. In
addition, this UART emulation uses the DMA to minimize CPU usage.
This application note provides a basic example of communications between a hardware and
a software UART as well as a summary of CPU load and firmware footprint.
The STSW-STM32156 firmware package is delivered with this document and contains the
source code of the UART emulator with all firmware modules required to run the example.
Table 1. Applicable products
Type
March 2015
Part numbers
Microcontrollers
STM32F4 series
Embedded software
STSW-STM32156
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1
Contents
AN4457
Contents
1
UART emulator description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
UART emulator block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3
UART emulator principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4
1.5
2
Single-frame transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3.3
Multiple-frame transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Data reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4.1
Single-frame reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4.2
Multiple-frame reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Baudrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Implementation structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.2
Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1
Frame transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.2
Transmission routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.1
Frame reception . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.3.2
Reception routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
UART emulator peripherals and main functions . . . . . . . . . . . . . . . . . . . . 16
2.4.1
Peripheral settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.2
Initialization and configuration function . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4.3
UART main functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.1
Hardware requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2
Software settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3
Running the example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.4
Frame waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
UART emulator CPU load and footprint . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1
2/24
1.3.2
2.1
2.4
4
Data transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Software description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3
3
1.3.1
CPU load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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Contents
4.2
UART emulator memory footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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List of tables
AN4457
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
4/24
Applicable products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Transmission/reception functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Callback functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
UART Emulator and UART hardware connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
UART CPU Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
UART memory footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
UART emulator block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9-bit data transmission waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9-bit data reception waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
UART emulator application level view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Frame transmission routine flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Transmission routine flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Frame reception routine flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Reception routine flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
UART emulator and UART HW connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Example MDK-ARM workspaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
UART emulator frame with no parity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
UART emulator frame with odd parity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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UART emulator description
1
AN4457
UART emulator description
The principle consists in emulating the UART protocol via GPIO, EXTI, timer and DMA
peripherals.
1.1
Main features
The main features of the UART emulator are the following:
•
Full-duplex asynchronous communications up to 115200 bps
•
Programmable data word length: from 5 to 9 bits
•
Flexible GPIO usage: all GPIOs can be configured as UART_TX/RX
•
Configurable number of stop bits: 1 or 2 stop bits
•
Parity control
•
•
6/24
–
Transmission of parity bit
–
Parity check of received data frame
Transfer detection flags
–
Receive complete
–
Transmit complete
Error detection flags
–
Frame error
–
Parity error
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UART emulator block diagram
Figure 1: UART emulator block diagram gives an overview of the interaction between the
hardware peripherals and the software modules that make up the UART emulator:
•
Software modules
They include the routines used for transmitting/receiving data and for formatting the
data to be sent by DMA or stored into SRAM. Flags indicating the UART status are also
available:
–
TC: Transmission Complete
This flag is set by software when data transmission is complete.
–
RC: Reception Complete
This flag is set by software when data reception is complete.
–
FE: Framing Error
This flag allows frame error detection.
–
PE: Parity Error
This flag is set by software when a parity error occurs in receive mode.
•
Hardware modules
They consist of all the STM32F4 peripherals involved in UART emulation:
–
GPIO: I/O ports used as transmitter or receiver pins.
–
EXTI: in reception mode, the start bit falling edge is detected via an EXTI.
–
DMA: data transfers are performed by DMA.
–
TIM: it ensures that data are transmitted and received at the required speed.
–
SRAM: data are formatted and stored in the SRAM.
Figure 1. UART emulator block diagram
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UART emulator description
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1. The status flags and the error flags are shown in green and red, respectively.
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UART emulator description
AN4457
1.3
UART emulator principle
1.3.1
Data transmission
The transmitter can send 5-, 6-, 7-, 8- or 9-bit data frames depending on user data length
configuration.
Figure 2 shows the waveforms of a 9-bit transmit emulation.
Figure 2. 9-bit data transmission waveforms
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1.3.2
Single-frame transmission
The transmission sequence includes the following steps:
1.
The CPU formats the frame to be sent to the memory.
2.
The timer sends to the DMA a request to set GPIO to 0 (start bit).
3.
The timer starts counting the start bit duration according to the defined baudrate.
4.
When the bit duration has elapsed, the timer sends to the DMA a request to set the
next bit from memory.
Once the frame transmission is complete, the TC flag (UART transmission complete) is set.
1.3.3
Multiple-frame transmission
Multiple -frame transmission is based on two buffers. When the DMA transfers the first
frame into the first buffer, the CPU formats the second frame in the second buffer. After each
frame transfer, the DMA TC flag must be cleared and the DMA reconfigured with the new
source address (address of first or second buffer). This operation is repeated until all frames
are transmitted. When this is done, the TC flag (UART transfer complete flag) is set.
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1.4
UART emulator description
Data reception
The reception is launched when the start bit is detected via an external interrupt on UART
RX line.
Figure 3 shows the waveforms of a 9-bit receive emulation.
Figure 3. 9-bit data reception waveforms
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1.4.1
Single-frame reception
Reception starts when the start bit is detected. The reception sequence includes the
following steps:
1.
The CPU checks if the UART is ready and the RX buffer empty.
2.
The timer sends a request to the DMA to start data transfer after a half bit duration.
3.
The timer starts counting the bit duration according to the defined baudrate and then
sends to the DMA a request to transfer the next bit. This step is repeated until the end
of the frame.
Once the frame reception is complete, the RC flag (UART reception complete) is set.
1.4.2
Multiple-frame reception
When the falling edge of the start bit is detected, the reception goes on until all frames are
received and the RC flag is set.
When the DMA transfer of each frame is complete, the CPU formats data and stores them in
SRAM.
Note:
During DMA transfer, the CPU is free for other tasks.
The application must assign the highest priority to the external interrupt for bit start
detection.
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UART emulator description
1.5
AN4457
Baudrate
The UART emulator supports baudrates up to 115200 bps.
The baudrate generation is ensured by the timer using different parameters such as clock
division, prescaler and period (ARR value).
The calculation of timer period (bit duration) is based on the following formula:
timer_period = timer frequency ⁄ UART baudrate
Example
•
Configuration
Timer prescaler = 0
Timer clock division = 0
AHB clock = system clock = 84 MHz
APB2 frequency = 21 MHz
UART baudrate = 9600 bps
•
Result
APB2 prescaler = 4
as a result
timer frequency = ( 84 ⁄ 4 ) × 2 = 42 MHz
and
timer period = 42000000 ⁄ 9600
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Software description
2
Software description
2.1
Implementation structure
Figure 4 shows the UART emulator software structure starting from application level.
Figure 4. UART emulator application level view
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Software description
AN4457
2.2
Transmission
2.2.1
Frame transmission
Figure 5 describes the implementation of the function that sends a give number of bytes
using DMA and timer.
TxXferCount is a counter that is incremented after the completion each DMA transfer.
FirstBuffer_Tx and SecondBuffer_Tx: the DMA source address switches between two
buffers addresses. The first buffer is used by DMA as source data, while the second buffer is
used by the CPU to format data.
Figure 5. Frame transmission routine flowchart
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2.2.2
Software description
Transmission routine
Figure 6 gives an overview of the UART emulator transmission routine.
Figure 6. Transmission routine flowchart
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Software description
AN4457
2.3
Reception
2.3.1
Frame reception
Figure 7 describes the implementation of the function that receives a given number of bytes
via DMA and timer.
RxXferCount: is a counter that is incremented after the completion each DMA transfer.
FirstBuffer_Rx and SecondBuffer_Rx: the DMA destination address switches between
two buffers addresses. The first buffer is used by DMA as data transfer destination, while
the second buffer is used by the CPU to format data.
Figure 7. Frame reception routine flowchart
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2.3.2
Software description
Reception routine
Figure 8 flowchart gives an overview of the UART emulator reception routine.
Figure 8. Reception routine flowchart
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Software description
AN4457
2.4
UART emulator peripherals and main functions
2.4.1
Peripheral settings
This section describes the configuration of the peripherals used inside the emulator.
•
•
•
•
GPIO
–
BSRR and IDR are used as destination and source registers for DMA transfers.
–
Two pins must be configured as input and output by the user application.
–
The input pin mode is configured as an EXTI line with falling edge detection.
DMA2
–
Channel 6 and stream 1 are used for transmission.
–
Channel 6 and stream 2 are used for reception.
–
The transfer is performed by words.
–
DMA transfer complete interrupt is used at the end of frame transfers.
Timer 1
–
Timer channel 1 is configured as capture compare for DMA transmit requests.
–
Timer channel 2 is configured as capture compare for DMA receive requests.
–
No clock division: CKD[1:0] =00.
–
No prescaler: PSC[15:0]=0.
SRAM
Four SRAM buffers are used to format data:
2.4.2
–
uint16_t pFirstBuffer_Tx[12] and uint16_t pSecondBuffer_Tx[12] are the buffers
for formatting data in transmission mode.
–
uint16_t pFirstBuffer_Rx[12], uint16_t pSecondBuffer_Rx[112 are the buffers for
formatting data in reception mode.
Initialization and configuration function
The initialization of the UART is performed by HAL_UART_Emul_Init function which allows
to:
•
setup the following UART parameters:
–
16/24
Baudrate
–
Frame length
–
Stop bit
–
Parity.
•
enable clocks for all the peripherals used: Timer, DMA, GPIOs
•
configure the DMA: channel, stream, mode, TC interrupt,...
•
configure the Timer: channel, period,...
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2.4.3
Software description
UART main functions
This section provides a set of functions ensuring UART transmission/reception emulation.
Table 2. Transmission/reception functions
Function
Parameters
HAL_UART_Emul_Transmit_DMA
(UART_Emul_HandleTypeDef *huart, uint8_t *pData,
uint16_t Size)
Description
huart: UART Emulator handle
pData: Pointer to data buffer
Size: Amount of data to be sent
HAL_StatusTypeDef HAL_UART_Emul_Receive_DMA huart: UART Emulator handle
(UART_Emul_HandleTypeDef *huart, uint8_t *pData,
pData: Pointer to data buffer
uint16_t Size)
Size: Amount of data to be received
Sends data
Receives data
Callback functions are also available. They allow the user to implement his own code in the
user file.
Table 3. Callback functions
Function
Parameters
Description
__weak void HAL_UART_Emul_TxCpltCallback
(UART_Emul_HandleTypeDef *huart)
huart: UART Emulator handle
This function is called at the end
of transmit process.
__weak void HAL_UART_Emul_RxCpltCallback
(UART_Emul_HandleTypeDef *huart)
huart: UART Emulator handle
This function is called at the end
of receive process.
__weak void HAL_UART_Emul_ErrorCallback
(UART_Emul_HandleTypeDef *huart)
huart: UART Emulator handle
This function is called when a
communication error is detected.
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Example
3
AN4457
Example
The example provided in STSW-STM32156 illustrates data exchange between the UART
emulator and the hardware UART.
3.1
Hardware requirements
The hardware required to run this example is the following:
•
Two Nucleo boards (NUCLEO-F401RE)
•
Two Mini-USB cables to power the boards and to connect the Nucleo embedded STLINK for debugging and programming.
The connection between the two Nucleo boards through UART lines is described in Figure 9
and Table 4.
Figure 9. UART emulator and UART HW connection
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Table 4. UART Emulator and UART hardware connection
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Nucleo board A (UART emulator)
Nucleo board B (hardware UART)
UART TX (PC10)
UART RX (PA10)
UART RX (PC11)
UART TX (PA9)
GND
GND
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3.2
Example
Software settings
The project example includes two workspaces: UART_EMUL and UART_HW (see
Figure 10).
To make the program work, follow the steps described below:
1.
Open your preferred toolchain (EWARM or MDK-ARM).
2.
Rebuild all files and load your image into target memory.
3.
Run the example.
Figure 10. Example MDK-ARM workspaces
3.3
Running the example
To run the example follow the sequence below:
Note:
1.
Power on the two boards.
2.
Load the code in each board MCU.
3.
Press the user button key on board A. The example then starts running and the UART
emulator starts transmitting data.
4.
The UART hardware receives the data and sends them back to UART emulator.
5.
The data transmitted by the UART emulator is compared to received ones: if data do
not match, the green LED (LED2) toggles continuously.
For more details, refer to the readme.txt inside the firmware package.
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Example
3.4
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Frame waveforms
Figure 11 and Figure 12 show examples of configurations of ‘A’ character transfer:
•
First configuration (see Figure 11):
–
Baudrate: 9600 baud
–
Word length: 8 bits
–
Number of stop bit: 1 stop bit
–
Parity: none.
Figure 11. UART emulator frame with no parity
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•
Second configuration (see Figure 12):
–
Baudrate: 9600 baud
–
Word length: 8 bits
–
Number of stop bit: 1 stop bit
–
Parity: parity odd.
Figure 12. UART emulator frame with odd parity
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6WRS
ELW
6WDUW
ELW
3DULW\ELW
06Y9
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UART emulator CPU load and footprint
UART emulator CPU load and footprint
The UART emulator uses the CPU for several tasks such as data formatting, DMA interrupt
handling both for transmission and reception and EXTI interrupt handling for reception.
4.1
CPU load
The CPU load depends on whether the transmit or receive process is active. When UART
full-duplex mode is active, the CPU load is increased. Refer to Table 5 for an example.
The software settings used to obtain the results given in Table 5 are the following:
•
System clock: 84 MHz
•
Toolchain: MDK-ARM V5.14, optimization level3(-O3) for size
•
Word length: 8 bits
•
One stop bit
•
No parity.
Table 5. UART CPU Load
Transmission
Reception
Baudrate (bps)
4.2
Load CPU (%)
MIPS
Load CPU (%)
MIPS
4800
2.4
2
4
3
9600
5
4
8.8
7
115200
6
5
9
7.5
UART emulator memory footprint
Table 6 gives an estimate of the code size required by the UART emulator software
compiled with MDK-ARM V5.14, optimization level3(-O3) for size.
Table 6. UART memory footprint
Flash memory footprint (bytes)
RAM footprint (bytes)
2872
400
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Conclusion
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Conclusion
This application note demonstrates that the product capability can be increased by adding
an emulated UART.
This solution has many advantages such as saving CPU usage by using DMA for data
transfer. In addition, the user can configure any GPIO as UART transmitter or receiver.
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Revision history
Revision history
Table 7. Document revision history
Date
Revision
30-Mar-2015
1
Changes
Initial release.
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